This invention relates to a gripper finger for a gripper spindle in a sheet-fed printing press.
German Patent No. 3,632,768.C1 and corresponding U.S. Pat. No. 4,873,926 disclose a gripper finger of the above type which includes a first compression spring having an adjustable spring force exerted approximately in the direction of a transfer point of a gripper support, and an adjustment screw whose tip lies against a supporting plane which also exerts a force approximately in the direction of the transfer point. The components are arranged so that the two forces sum to form a resultant force which has a direction coinciding with a vector which approximately connects the center point of the gripper spindle and the transfer point. This arrangement, which insures that the force resulting during operation extends approximately in a line which connects the center of the gripper spindle and the transfer point of the gripper support, gives gripper fingers of this type the significant advantage that practically no change of play occurs in the various operating states.
Such gripper fingers, however, are subject to a relatively great amount of wear, and replacing gripper fingers is an expensive operation. The gripper spindle must be removed as a whole before the worn gripper fingers can be removed from the gripper spindle. This is labor intensive and also leads to presently unacceptably high down times of the machine.
German Patent No. 3,623,405 and corresponding U.S. Pat. No. 4,781,370 disclose a gripper finger which is relatively easy to replace. This gripper finger is made of two parts, a lower gripper finger component and an upper gripper finger component directly thereabove. The ends of the components adjacent to the transfer point are spread apart by a compression spring, and an adjustment screw is provided at the other ends to regulate the distance between the two gripper finger components. Both gripper finger components are fastened to the gripper spindle by means of a screw which passes through a hole in the upper gripper finger component and is screwed into a thread in the gripper spindle. The lower gripper finger component is provided with a recess which has a partially cylindrical profile such that it places itself against the gripper spindle from the same side (upper side) where the upper gripper finger component is also disposed.
Although this prior art gripper finger has the advantage that installation and removal merely require adjustment of the fastening screw, a great change in play results when the gripper is opened and closed and consequently a displacement of the gripper tip results, with the degree of the change in the play and the displacement being dependent on the wear of the gripper bearing. This wear and change in play result in doubling of sheets.
German Patent No. 1,174,804 discloses a gripper finger having a body which covers an opening of about 180 degrees so that installation of this gripper finger also does not require removal of the gripper spindle. The opened body of the gripper finger is closed by a spring with an adjustable spring force. However, this gripper finger still has the above described disadvantages of wear and change in play, particularly because the spring for closing the opening does not pose sufficient resistance in this connection. There is also no provision for realizing the above described important feature of having the forces generated by a compression spring and a contact face combine so that the resultant of these two forces connects the center point of the gripper spindle with the transfer point.